Oxygen-based biomass combustion system and method

ABSTRACT

An oxygen-based biomass solid fuel combustion system and method has an air separator for separating oxygen from air providing a supply of oxygen for feeding oxygen to a solid fuel combustion chamber. An airlock feeds a metered amount of solid fuel to the solid fuel combustion chamber. A burner stage having a firetube for collecting fuel gases from the solid fuel combustion chamber combusts the collected fuel gases with further oxygen from the separator and heats a boiler to generate steam. A heat utilization device (e.g. an electrical generator) may be connected to the steam boiler. Nitrogen-free diluent gases (e.g. argon and carbon dioxide) are used to control combustion process temperatures. The usable heat energy and useful byproducts are extracted from the different stages of the process.

REFERENCE TO RELATED APPLICATION

The present application is the subject of provisional application No.60/458,377 filed Mar. 31, 2003 entitled WASTE ENERGY SYSTEM WITH ZEROEMISSION. This application is also a continuation-in-part application ofapplication Ser. No. 09/138,020 filed Aug. 21, 1998 entitled GASIFIERSYSTEM AND METHOD (now abandoned) and application Ser. No. 10/061,362filed Feb. 4, 2002, now U.S. Pat. No. 6,532,879 and acontinuation-in-part application of application Ser. No. 10/331,559filed Dec. 31, 2002.

BACKGROUND AND BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to a gasifier system and method for theefficient conversion of solid fuel waste materials (tires, etc.) andbiomass materials into usable energy and useful products.

DESCRIPTION OF THE PRIOR ART

The prior art is best exemplified by U.S. Pat. No. 5,284,103 entitledBIO-MASS BURNER CONSTRUCTION by Hand et al, which is a division of U.S.Pat. No. 5,178,076 entitled BIO-MASS BURNER CONSTRUCTION issued Jan. 12,1993 to the same inventors. In these patents, the burner utilizes afirst burning chamber having a falling fuel entrained bed zonepositioned above a traveling grate having a porous metallic woven belt.Primary air is directed through the porous belt to establish anoxygen-starved first burning chamber. A second burning chamber in fluidcommunication with the first burning chamber has a restricted diameterand effectively provides a hot-air gas nozzle. In larger sized units, aplurality of conveyors constitutes the traveling grate with theconveyors being arranged in head-to-head stepped relationship so thatunburned fuel received by gravity from the entrained bed zone isagitated or jostled to enhance its burning.

Reference is made to the following prior art patents: 4,749,383Mansfield et al. 4,385,567 Voss 5,279,234 Bender et al. 3,853,498 Bailie5,589,588 McMullen et al. 4,109,590 Mansfield 4,244,180 Rasor 4,276,120Lutz 4,448,599 Cheng 4,624,192 Mansfield 4,829,911 Nielson 4,838,183Traveras et al. 5,081,940 Motomura et al. 5,105,747 Khinkis et al.5,588,378 Mancini 5,727,482 Young

BRIEF SUMMARY OF THE INVENTION

According to the present invention, firebelts ensure that the heat lossfrom heating unnecessary oxidant is minimized. The quantity of oxygen ateach point in the combustion process is closely controlled. This oxygencontrol benefits the combustion process in three ways:

First, by minimizing the heat loss of the combustion process, thisminimizes the amount of carbon monoxide and volatile organic carbons(VOC) that are produced. Carbon monoxide and VOC, priority pollutants,are produced indirectly proportional to the combustion temperature.Therefore, by maximizing the combustion temperature, the quantity ofcarbon monoxide and VOCs produced are minimized.

Secondly, nitrogen oxides, another priority pollutant, is produced bycombining the nitrogen in the air with the oxygen. This combination ofnitrogen and oxygen only occurs at high temperatures and when nitrogenis present. The higher the temperature and the greater the nitrogencontent of the gases, the greater the quantity of nitrogen oxides thatare produced. While the combustor of the present invention utilizes hightemperatures, the formation of nitrogen oxides is eliminated since thereis no nitrogen in the oxidant and diluent to combine with the oxygen.All the oxygen is used in the combustion process.

Thirdly, by minimizing the amount of air supplied to the combustionprocess, the amount of energy required to move gas in the combustor andancillary is minimized. Electrical energy costs are typically 20-50%less than similar combustion systems where the air is used.

A further feature of the present invention is in the use of reflectedinfrared energy. Heat is a form of electromagnetic energy similar tolight where the rays can be refracted or reflected. Radiation producedfrom heat is of a longer wavelength than visible light and is calledinfrared rays. By reflecting a certain amount of the heat produced froma combustion process, this invention is able to supply heat to thegasification process. The reflected heat will be of benefit in two ways:

Firstly, the heat is reflected to a point where the heat can be used toassist the combustion process. This is generally where the fuel firstenters the combustion process. At this point, the fuel must be heatedand the water removed. These processes require addition of energy thatcan be added for heat of the combustion of a part of the fuel or fromthe reflected energy. Using a part of the fuel to preheat the remainingfuel is inefficient, leaving less total heat available for production ofelectricity. Using reflected heat removes or minimizes thisinefficiency. The second way this benefits the overall combustionprocess is in that the energy is transferred in a beneficial way—notwasted by irradiating and heating the combustion chamber. Heat that isabsorbed by the combustion chamber shell is generally wasted since thereis no direct benefit from this radiation. A small portion is used in themaintenance of the necessary combustion temperature but the majority ofirradiating heat is wasted as low-level heat irradiating from thecombustor exterior. Reflective heat added to the fuel will benefit theoverall combustion efficiency, and this is a feature of the presentinvention

Another feature of the invention is that the speed of the conveyor driveand the rate of inlet oxidant and diluent addition and the control ofthose gases is much more closely controlled so as to achieve highefficiency. Still another feature of the invention is that the fuel feedramping is based on thermal conditions at the boiler output.

Still another feature of the invention is that the induced draft fancontrol is based on the draft pressure and boiler airflow rate.

Finally, the invention features a control system which is based onoperational parameters sensed at different stages in the process.

Accordingly, the object of the invention is to provide an improvedgasifier system.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and features of the inventionwill become more apparent when considered with the followingspecification and accompanying drawings wherein:

FIG. 1 is a diagrammatic sectional illustration of a gasifier systemincorporating the invention,

FIG. 2 is a diagrammatic illustration of an air separator unitincorporated in the invention to separate and purify the oxygen used asan oxidant in the combustor unit.

FIG. 3 is a cross-sectional illustration of a walking floor trailer andinclined conveyor incorporated in the invention for feeding fuel to thecombustor unit,

FIG. 4 is a diagrammatic illustration of the bin feeder and rotary airlock incorporated in the invention,

FIG. 5 is a diagrammatic illustration of the boiler for convertingthermal energy to steam,

FIG. 6 is a diagrammatic illustration of the cyclone and baghouseparticulate collection division,

FIG. 7 is a diagrammatic illustration of the scrubber for removing acidgases from the gas stream,

FIG. 8 is a diagrammatic illustration of the carbon dioxide scrubber forremoving carbon dioxide from the gas stream, and

FIGS. 9A, 9B and 9C are flow diagrams illustrating the combustionprocess and location of various sensors and control elementsincorporated in the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, the gasifier system comprises an air separatorunit 10, gas storage for oxygen 11, nitrogen 12, argon 13, a solid fuelstorage unit 14 and fuel transport conveyor 15, inlet feed conveyor 16and fuel metering unit 17, a gasifier 18, a gasifier firetube withconnection to boiler 19, the boiler itself 20, a cyclone 21, baghouse22, acid gas scrubber 23, carbon dioxide scrubber 24, carbon dioxidestorage system 25, and control system 26 which is diagrammaticallyillustrated in FIGS. 9A, 9B and 9C.

Air Separator Unit 10 (FIG. 2)

The air separator unit 10 separates air into its components parts;oxygen, nitrogen and argon. Trace gases normally present in air arevented. Gaseous air is compressed by a turbine compressor 2-10 andpasses into condensing column 2-11 where the compressed gases are cooledand condensed into liquids. The liquids then pass into a fractionatingtower 2-12 where they are heated and separated by their boiling points.The separated, purified gases are collected in pressure vessel 11 aspurified oxygen, pressure vessel 12 as purified nitrogen and pressurevessel 13 as purified argon. The process uses the oxygen while thenitrogen and argon are available as industrial commodities.

Walking Floor Trailer 14 and Inclined Conveyor 15 (FIG. 3)

The walking floor trailer 3-10 is used to store and deliver fuel to thecombustor with the inclined conveyor. The bin conveyor 3-11 monitors theamount of fuel and calls for additional fuel from the walking floortrailer when necessary.

The walking floor trailer 3-10 is conventional and works in thefollowing manner: The floor 3-13 is comprised of a number of strips thatindependently move. To move the fuel to the rear of the trailer (leftend in FIG. 3), the strips all move together rearward. At the end of thecycle (approximately four inches), each strip independently movesforward, leaving the fuel undisturbed. This cycle is repeated asrequired to move the fuel rearward as much as necessary. A monitor orsensor 3-14 is located at the input end of inclined conveyor 15 andmonitors the quantity of fuel located at the end of the trailer (thebeginning of the inclined conveyor) and moves the fuel rearward in thewalking floor trailer as required to maintain sufficient fuel in theinclined conveyor. Sensors 3-15, 3-16 and 3-17, similar to sensor 3-14are used to monitor fuel quantity along the length of the walking floortrailer to insure there is sufficient fuel in the trailer. ps Bin Feeder16 and Rotary Air Lock 17 (FIG. 4)

The bin feeder 4-10 receives the fuel from the inclined elevator 15(FIG. 3). The bin feeder is used to meter the fuel to the combustor.Depending upon the quantity of fuel required by the combustor to producesufficient steam, the bin feeder 4-10 speed will be varied to introducesufficient fuel to the combustor. A monitor or sensor 4-12 is used onthe bin feeder to ensure that sufficient fuel is present on the binfeeder belt. If additional fuel is needed, a signal is sent to theinclined conveyor/walking floor trailer (FIG. 3) for additional fuel.Load cells 4-13 are used to calculate the fuel weight per unit time thatis introduced to the combustor.

The rotary air lock 7 is used to provide a mechanical seal in chute 4-11to minimize the quantity of unwanted air introduced into the combustor.

Gasifier 18

The fuel gasification process takes place in gasifier 18 shown in FIG. 1device. This process is described hereafter in the section entitled“COMBUSTION PROCESS DESCRIPTION”. The area of this gasification isreferred to as stage 1. The gasification in this stage is controlledtotally by the control system by taking the following parameters and thesystem computer 26 determines the variables that need to change orremain the same during the gasification process:

Stage 1 a:

-   -   Solid fuel feed rate,    -   Oxidant flow rate,    -   Diluent flow rate,    -   Firebelt 30 speed,    -   Gasification temperature,    -   Draft pressure,    -   Infrared flame strength,    -   Ultraviolet flame strength.        Stage 1 b:    -   oxidant flow rate,    -   Diluent flow rate,    -   Firebelt 31 speed,    -   oxidation temperature,    -   Draft pressure,    -   Firebelt (Conveyor) Air Flows,    -   Infrared flame strength,    -   Ultraviolet flame strength.

The pure oxygen and carbon dioxide as a diluent through the firebelts(conveyors) 30, 31 is the primary reason for the ability to eliminateair emissions from the combustor disclosed herein. Controlling theamount of oxidant 5-11 (firebelt-1 30), 5-13 (firebelt-2 31) and diluent5-12 (firebelt-1 30), 5-14 (firebelt-2 31) that passes through each unitarea of the firebelts governs the quantity and quality of the combustionprocess.

The combustion process requires two components; fuel and an oxidizer.Normally, air is used as an oxidizer but air contains many gases, mostof which do not contribute to the oxidation process. In fact, the othergases in air can have a deleterious effect on the overall combustionefficiency and the production of air pollutants emitted to theatmosphere. Therefore, only pure oxygen is used as an oxidant and carbondioxide is used as a diluent to eliminate the potential for formation ofuntreatable air pollutants.

The entire combustion process is a series of discrete steps where heatis either created or it is used. In general, the following steps occurin the solid fuel combustion process: Firebelt 30 Heat solid fuel tooperating requires energy temperature Remove adsorbed water from solidfuel requires energy Heat combustion air to operating requires energytemperature Gasify fuel components in solid fuel requires energyDecompose fuel gases into elemental requires energy fuels Reflected heatfrom firebelt 21 produces energy Combust small percentage of solid fuelproduces energy for heat Net energy: produces very little energy Heatloss: negligible Firebelt 31 Heat combustion air to operating requiresenergy temperature Additional heating of fuel and ash from requiresenergy 1,500. degree. F. to 2,500. degree. F. Reflect heat to firebelt20 requires energy Combust carbon into carbon monoxide produces energyPreheat air from hot bottom ash produces energy Net energy: produceslittle energy Heat loss: small - hot bottom ash Firetube 19 Heatcombustion air to operating requires energy temperature Additionalheating of fuel and ash from requires energy 2,500. degree. F. to 4,000.degree. F. Combust fuels produces energy Preheat combustion air fromcombustor produces energy refractory Net energy: produces significantenergy Heat loss: negligible Boiler Superheater Heat transferred tosteam requires energy Net energy: requires significant energy Heat loss:negligible Boiler 11 Heat transferred to boil water requires energy Netenergy: requires significant energy Heat loss: negligible BoilerPreheater Heat transferred to water requires energy Net energy: requiressignificant energy Heat loss: negligible Cyclone 21 Heat lost viaradiant heating produces energy Heat lost in fly-ash produces energy Netenergy: produces unusable energy Heat loss: significant Baghouse 22 Heatloss via radiant heating produces energy Heat lost in fly-ash producesenergy Net energy: produces unusable energy Heat loss: significant AcidGas Scrubber 23 Heat loss via radiant heating produces energy Heat lostin quenching gas with caustic produces energy solution Net energy:produces unusable energy Heat loss: significant Carbon Dioxide Scrubber24 Heat loss via radiant heating produces energy Heat loss viacompressive heating produces energy Heat lost in cryogenic chillingproduces energy Net energy: produces unusable energy Heat loss:significant Heat Balance Heat Produced     100% Heat used to producesteam     88% Heat loss through radiant convection     10% Heat lossfrom ash      2% Heat loss through combustor, fire tube <0.001% andboiler

To ensure that the heat loss from heating unnecessary oxidant anddiluent is minimized, the quantity of oxygen and carbon dioxide at eachpoint of the combustion process is stringently controlled. This controlbenefits the combustion process in three ways.

The first way is by minimizing the heat loss in the combustion process.This minimizes the amount of carbon monoxide produced. Carbon monoxide,a priority pollutant, is produced indirectly proportional to thecombustion temperature. Therefore, by maximizing the temperature whilemaintaining a slight excess of oxygen, the quantity of carbon monoxideis minimized to the point of non-detection. Second, nitrogen oxides,another priority pollutant, is produced by combining the nitrogen in airwith the oxygen in the air. This combination of nitrogen and oxygen onlyoccurs at high temperatures and when nitrogen is present. The higher thetemperature, the greater the quantity of nitrogen oxides that isproduced and the greater quantity of nitrogen present, the greater thepotential for nitrogen oxide formation. While the combustor disclosedherein utilizes very high temperatures, the formation of nitrogen oxidesis eliminated since there is no nitrogen to combine with the oxygen. Allof the oxygen is used in the combustion process.

Third, by minimizing the amount of oxidant and diluent supplied to thecombustion process, this also minimizes the amount of energy required tomove the gases in the combustor. Electrical energy costs are typically20-50% less than similar combustion systems where the air is used.

Reflection of Infrared Energy

Heat reflection is another innovative feature of the combustor of thisinvention. Heat is a form of electromagnetic energy, similar to visiblelight where the rays can be refracted or reflected. Radiation producedfrom heat is of a longer wavelength than visible light and is calledinfrared rays.

By reflecting a certain amount of the heat produced from the combustionprocess, additional heat is supplied to the gasification process. Thereflected heat will be of benefit in two ways:

The first way is that by the heat reflected to a point where the heatcan be used to assist the combustion process. This is generally wherethe fuel first enters the combustion process. At this point, the fuelmust be heated and the water removed. These processes require theaddition of energy that can be added from either the combustion of apart of the fuel or from the reflected energy. Using a part of the fuelto preheat the remaining fuel is inefficient, leaving less total heatavailable for the production of electricity. Using the reflected heatremoves or minimizes this inefficiency.

The second way reflected heat benefits the overall combustion process isthat the energy is transferred in a beneficial way and is not wasted byirradiating and heating the combustion chamber shell. Heat that isabsorbed by the combustion chamber is generally wasted since there is nodirect benefit from this radiation. A small portion is used in themaintenance of the necessary combustion temperature, but the majority ofthe radiative heat is wasted as low level heat radiated from thecombustor exterior. The reflection of the heat back onto the fuel willbenefit the overall combustion efficiency.

Gasifier Firetube with Connection to Boiler 19

This is actually a part of the gasifier and is referred to as stage 2.It is the connecting tube to the boiler 20. Oxidant and diluents 5-15and 5-16 are preheated in annular spaces 5-18 in the refractory of thegasifier and injected at a rate dictated by the control system. Whenthis oxygen meets the gas from the gasifier, ignition takes place in thefiretube and thus enters the boiler. The following parameters are takenin the firetube for the control system's use:

Stage 2:

-   -   Oxygen and diluent flow rate,    -   Oxygen concentration,    -   Carbon monoxide concentration,    -   Firetube draft pressure,    -   Firetube temperature,    -   Boiler draft pressure,    -   Boiler temperature.        Boiler 20 (See FIG. 9B)

The boiler 20 (FIG. 5) converts the thermal energy to steam. Thefollowing parameters (see FIG. 9B) are taken in the boiler for thecontrol system 26 use for control of the feed rate and the steam output:

-   -   Oxygen and diluent flow rate,    -   Steam pressure,    -   Steam temperature.

The boiler 5-10 (20) used in this embodiment of the invention is aD-frame boiler. Other types of boilers such as A-frame, H-frame may beused in other installations.

The boiler 5-10 absorbs the heat produced from the combustion of thefuel and transfers it to water, which is converted into steam. The steamis used to produce mechanical work such as electrical generation,heating, etc.

An economizer may be attached after the boiler to preheat the water andimprove efficiency. This is not shown in this embodiment.

Cyclone 21 & Baghouse 22

The cyclone 21 (FIG. 6) is the first mechanical device that removesparticulate. The design of the cyclone is such that when the exhaustgases flow through it from the boiler the largest particulate will dropfrom the flow through the bottom of the cyclone to a storage container.The following parameters are measured (see FIG. 9B) for the controlsystem 26 in the cyclone:

-   -   Inlet temperature 6-12,    -   Inlet pressure 6-13,    -   Outlet temperature 6-14,    -   Outlet pressure 6-15.

The cyclone outlet temperature and pressure are also used for thebaghouse inlet.

The cyclone 6-10 and baghouse 6-11 operate as particulate collectiondevices. In the combustion process, as the fuel is combusted a smallpercentage of ash remains. Some of this ash is entrained within thecombustion gas stream and is carried along with the exhaust gases,called fly-ash. The cyclone 6-10 and baghouse 6-11 remove the fly-ash sothat it is not emitted into the atmosphere.

The cyclone acts through centripetal action. The gas spins around in thecyclone and separates the heavy particles from the gas based uponweight. The ash particles are collected at the bottom of the cyclone andremoved through a rotary air lock 6-18 and a vacuum removal system(eductor) 6-19.

The baghouse 6-11 operates on a different principle. The gas passesthrough a series of fiberglass bags 6-22 that have very small openingswithin them. The gas can pass through but the particles cannot andremain collected on the exterior of the bag. At appropriate intervals,high-pressure air is introduced inside the bags. This air literallyforces the particles off of the bags and they fall to the bottom of thebaghouse. The particles then are collected through a rotary air lock6-20 and an eduction system 6-21 similar to the cyclone.

The cycle acts through centripetal action. The particles are collectedon the surface of the bags and are removed by high-pressure air pulsemainfold 6-23 delivered countercurrent through the bag material whichcauses the adhered particles to fall off. The ash particles arecollected at the bottom of the baghouse and removed through a rotary airlock 6-20 and a vacuum removal system (eductor) 6-21.

The baghouse 22 (FIG. 6) is the final removal equipment for particulate.The mesh size of the bags will be determined by the particulatedischarge requirements. The following parameters are taken in thebaghouse for the control system use:

-   -   Inlet temperature 6-14,    -   Inlet pressure 6-15,    -   Outlet temperature 6-15,    -   Outlet pressure 6-17.

The baghouse inlet temperature and pressure are also used for thecyclone outlet.

Acid Gas Scrubber 23

The acid gas scrubber 7-10 (FIG. 7) is used to remove acid gases fromthe exhaust gas stream. A mixture of lime (calcium oxide, a strongcaustic) or sodium hydroxide (lye, a very strong caustic) and water issprayed 7-11 through the exhaust gas. This liquid chemically reacts withthe acid gases such as sulfur dioxide, hydrogen chloride, etc. to removethe acid gases. Plastic or ceramic open-frame balls 7-12 are often usedas packing to increase the surface area of the contact surface toimprove the efficiency of the chemical reaction. After the liquid hasreacted with the gas, the gas stream passes through a series ofimpediments, called demisters 7-13 to remove all excess liquid. Thecleaned gas then proceeds to the carbon dioxide scrubber.

After the liquid has reacted with the acid gases, it is collected in aspent slurry collector 7-14 and returned for treatment-to a source by apump where it is sent to a separation chamber and the caustic solutionrecycled.

The scrubber 7-10 (FIG. 7) cleans the acid gases from the air stream. Itis usually a wet or dry caustic system depending on the carbon dioxidescrubber requirements. The following parameters are taken for thecontrol system that then determines the feed rate for the caustic agent:pH 7-15

Cleaned exhaust gases consisting of almost pure carbon dioxide is fed byfan to the carbon dioxide scrubber 24.

Carbon Dioxide Scrubber 24 (FIG. 8)

The carbon dioxide scrubber 8-10 cryogenically removes carbon dioxidefrom the remaining gas stream. The acid gas scrubber exhaust iscompressed by a turbine compressor 8-11 and passes into condensingcolumn 8-10 where the compressed gases are cooled and condensed intoliquid carbon dioxide and trace amounts of other gases not collected bythe acid gas scrubber 23. The liquids then pass into a fractionatingtower 8-12 where they are heated and the purified carbon dioxideremoved. The separated, purified carbon dioxide is collected in pressurevessel 8-13. Trace amounts of acid gases and other inert gases arevented 8-14 and are returned to combustor 18 or the air separator unit10 for recycling.

Control System 26

The control system (see flow charts in FIGS. 9A, 9B and 9C) is thedetermining factor for the gasification system to operate properly andto be in compliance with the regulatory requirements for air discharge.It includes Programmable Logic Controllers (PLC's) and variable speeddrives, diagrammatically illustrated in FIGS. 9A, 9B and 9C, thatoperate the various motors, fans and drives that operate the gasifiersystem. The PLC'S are in turn controlled by signals from a computer thatis programmed to recognize all the variables listed plus other minoritems and to react properly from operator input. The program is designedto make adjustments for different types of fuels (with different BTUcontent) without changing equipment in the gasifier system.

Combustor Process Description

Thermodynamic Extraction of Chemical Potential Energy

The release of chemical potential energy is a two-stage process: Stage 1gasifies the carbon-based solid fuels and stage 2 oxidizes the gasifiedfuels to produce heat.

Stage 1 is subdivided into two separate processes involved in thegasification of solid fuels. Stage 1 a uses thermal decomposition of thesolid fuel introduced into the combustor to break the fuel into gaseousfractions of lower molecular weight or elemental composition. Allabsorbed compounds in the fuel such as water and other solvents arereleased in this stage.

A polymerized hydrocarbon based fuel (plastic and lignin/cellulose basefuel) is decomposed into short chain aliphatic hydrocarbons, elementalcarbon, carbon monoxide and hydrogen through the addition of energy asheat. Other elemental based polymers including sulfur and nitrogen basedcompounds are similarly broken into appropriate monomers or elementsusing the same process. The ash produced from Stage 1 a is largelycarbon char with small amounts of metal oxides.

The heat required for the endothermic decomposition of the fuel isproduced from heat supplied from stage 1 b and from limited oxidation offuel in stage 1 a. The oxidation of the solid fuel is limited in thisstage by careful addition of pure oxygen to the combustion process inStage 1 a. The amount of oxygen injected into Stage 1 a is controlled bythe amount of oxidation required to maintain the minimum necessarydecomposition temperature in this stage. Carbon dioxide is also added asa diluent to stage 1 a to carry the gasification products to stage 2 andto provide sufficient cooling of the process to prevent overheating ofthe belt and refractory. The amount of oxygen and carbon dioxideinjection in stage 1 is governed by the temperature of the gases exitingstage 1 a and stage 1 b.

Stage 1 b utilizes an exothermic partial oxidation of the carbon in theash to produce carbon monoxide and heat. This process is regulated byadding a sub-stoichiometric amount of pure oxygen to the carbon char tolimit the reaction of carbon and oxygen to the production of carbonmonoxide. Carbon dioxide is also added as a diluent to stage 1 b tocarry the resulting carbon monoxide to stage 2 and to provide sufficientcooling of the process to prevent overheating of the belt andrefractory. The remaining solid ash consists entirely of metallicoxides.

The heat of reaction of the carbon oxidation is used in Stage 1 a todecompose the fuel as previously described. Approximately 80% of theheat of reaction is utilized in this process with the remaining heatpassing to stage 1 b and stage 2.

Physical Process

Solid fuel is introduced to the combustor section 1 a (firebelt 30 )where gasification and moisture removal occurs. A minimal amount ofoxygen is introduced to Section 1 a to maintain the minimum gasificationtemperature necessary for the specific type of fuel used. Radiativeenergy from Section 1 b, firebelt 31 is also added to the energyrequired for gasification.

The gases exiting this section consist of primarily carbon monoxide,hydrogen, hydrocarbons (short-chain and long-chain), elementalfixed-carbon and water vapor with minimal quantities of carbon dioxide.Only fuel-bound nitrogen is present in the gases sincenitrogen-containing air is not used. The ash produced through thegasification process consists of carbon, long-chain, high-boiling-pointhydrocarbons and metallic oxides.

Control of the gasification process is accomplished by modulation of thefuel feed rate, the quantity of oxygen introduced through firebelts asmeasured by the gas temperature and the speed of the firebelts. Oxygeninjected into the solid fuel is minimized to prevent overheating of theoxygen/fuel reaction and to prevent complete oxidation of carbon tocarbon dioxide. The firebelt speed is controlled so that the solid fuelhas been completely gasified at the end of the belt and only carbonchars remain.

Carbon dioxide is injected along with the oxygen to act as a carrier ofthe gasification products and as a diluent to moderate the reactiontemperatures. Modulation of the carbon dioxide is controlled by thegasification temperature, the firebelt 30 temperature, and therefractory temperature.

The carbon ash from firebelt 30 falls onto section 1 b (firebelt 31)where additional oxygen, in decreasing quantities, is supplied tocombust the carbon to carbon monoxide as well as decomposition of thelong-chain hydrocarbons to carbon monoxide and hydrogen. Section 1 bgases consist of carbon monoxide (40-55%), hydrogen (5-20%) with thebalance of the gas being carbon dioxide. The ash remaining from thisprocess consists of metallic oxides with trace quantities ofcarbon-based compounds. Trace amounts of fuel-bound nitrogen, fuel-boundsulfur and inert gases are also present in variable quantities dependingupon the fuel composition. oxygen content is minimized in stage 1 b toprevent oxidation of the carbon to carbon dioxide. The firebelt speed iscontrolled to ensure complete oxidation of the ash just before the endof the belt.

Radiative energy produced from stage 1 b is reflected off of therefractory walls onto section 1 a where it is used to gasify the solidfuel. Control of the section 1 b process is performed by the firebelt 31speed, stage 1 b temperature, control of the air to fuel ratio throughfirebelt 31 as measured by the oxygen concentration and by the overalldraft (negative pressure) of the combustor system.

Stage 2 combustion occurs within the firetube and within the boilerwhere the carbon monoxide and hydrogen gases are oxidized to carbondioxide and water by the addition of additional oxygen. The firetube isused for mixing of the oxygen and fuel gas and preliminary combustionwith final combustion occurring within the boiler cavity.

The Stage 2 combustion process is controlled by the oxygen/fuel gasratio, boiler temperature, firetube temperature, carbon monoxideconcentration, oxygen concentration, carbon dioxide concentration and bythe overall draft of the combustor system. Carbon dioxide is also addedas a diluent to the combustion process to moderate the combustiontemperatures to prevent overheating of the refractory and boilercomponents. Carbon dioxide addition is controlled by the firetube andboiler temperatures.

Pollution Control

The combustor of the invention is a remarkably simple combustion system,and this design eliminates the emission of air pollutants from thecombustion of solid fuels. All potential air pollutants are removedbefore potential emission into the environment. This is not the casewith other combustor systems currently on the market.

There are six different categories of air pollutants that theEnvironmental Protection Agency regulates in solid fuel combustionsystems. The combustor disclosed herein has been specifically designedto eliminate each of these six categories of pollutants. Each of thesesix categories will be discussed individually.

Particulates

Particulates can potentially be released into the atmosphere frommaterials in the fuel which cannot be burned. Usually these particlesare a chemical part of the fuel and when burned, recombine as smallparticles. Part of these particles agglomerate together in chunks whichthen collect in the bottom of the combustor and are removed as bottomash. The remaining particles are carried in the flue gas.

These particles could be released into the atmosphere unless they areremoved. In the combustor of this invention, the particles are removedby devices called a cyclone and a baghouse. The cyclone acts throughcentripetal action. The gas spins around in the cyclone and the heavyparticles are separated from the gas based upon weight. The baghouse isa large chamber filled with cloth bags that collect the dust as the gaspasses through them. The dust is then removed and the cleaned gas isprocessed for other contaminants.

The technology of cyclone and baghouse design and construction is welladvanced. There have been very few refinements in the baghouseparticulate removal system since the mid-1970's.

Nitrogen Oxides

Nitrogen oxides (NO_(X)) are produced by nitrogen combining with oxygenin the presence of high temperatures. Generally, the higher thetemperature, the higher the quantity of nitrogen oxides produced.Because nitrogen oxides have been found to be a contributing factor inthe destruction of ozone in the atmosphere, the emission of thesecompounds are regulated and must be minimized.

For most solid fuel combustion sources, air is used as a source ofoxidant and therefore a system of reducing nitrogen oxides must be addedto lower the nitrogen oxide emissions to an acceptable level. Typicallythis reduction system uses the injection of ammonia gas into thecombustion system and a catalytic converter (similar to today'sautomobiles) to chemically react the nitrogen oxides and the ammonia toproduce nitrogen gas and water. This is an expensive process, both incapital costs for the precious metal catalyst and for operating costs ofammonia injection. Additionally, another pollutant, ammonia, a highlytoxic compound, can be introduced into the atmosphere that must becontrolled.

This combustor design uses an entirely different method to reducenitrogen oxides. By eliminating nitrogen in the combustion process, theformation of nitrogen oxides is eliminated. Nitrogen oxides cannot formif there is no nitrogen to combine with the oxygen. In the combustionprocess disclosed herein, only enough oxygen is added to the fuel toperform the necessary combustion of the fuel. Because there is nonitrogen, there are no nitrogen oxides produced. A small excess oxygenis added only at the very end of the combustion process to ensurecomplete combustion of the fuel. Using this process, very lowconcentrations of nitrogen oxides are produced since only fuel-boundnitrogen, generally only present in trace amounts, can produce nitrogenoxides.

Sulfur dioxide (SO₂) is produced by the fuel-bound sulfur combining withoxygen. Generally, the higher the concentration of sulfur in the fuel,the higher the quantity of sulfur dioxide produced. Because sulfurdioxide have been found to be a contributing factor in the formation ofacid rain in the atmosphere, the emission of these sulfur dioxide mustbe minimized or eliminated.

In this process, a commercially available caustic scrubber is used toremove the sulfur dioxide. A mixture of lime (calcium oxide, a strongcaustic) or sodium hydroxide (lye, a very strong caustic) and water issprayed through the exhaust gas. This liquid chemically reacts with theacid gases such as sulfur dioxide, hydrogen chloride, etc. to remove theacid gases. Plastic or ceramic open-frame balls are used as packing toincrease the surface area of the contact surface to improve theefficiency of the chemical reaction. After the liquid has reacted withthe gas, the gas stream passes through a series of impediments, calleddemisters to remove all excess liquid.

Acid gas scrubber technology is mature and well represented by manymanufacturers. Little has changed with this technology since the early1980s.

Carbon Monoxide

Carbon Monoxide (CO) is the result of incomplete combustion. This is dueto either low combustion temperatures or insufficient oxidation. In thecombustor of this invention, the combustion temperature exceeds 3,000°F. for all solid fuels and up to 4,000° F. using tires. To ensure thatthe combustion process is complete and no carbon monoxide remains, asmall amount of excess oxygen is added to the final stage of combustion.This results in negligible concentrations of carbon monoxide. Anyunreacted carbon monoxide passes through the entire pollution controlsystem and is reintroduced into the combustor as part of the diluentcarbon dioxide where complete oxidation of the carbon monoxide canoccur.

Volatile Hydrocarbons

Volatile hydrocarbons or volatile organic carbons (VOC) are a class ofcompounds that are regulated by the Environmental Protection Agency.These compounds include a wide range of chemicals that can be emittedinto the atmosphere. Included in this list are compounds like dioxins,polychlorinated biphenyls (PCBs), polynuclear aromatics (PNAS) and otherhazardous air pollutants (HAPS).

These compounds are created as the result of incomplete combustion. Inthe combustor of this invention, the formation of these compounds arekept to an extremely low level, in many cases unmeasurable due to theextreme temperatures present in the combustion process.

Carbon dioxide

Carbon dioxide (CO₂) is considered to be a greenhouse gas that has thepotential of affect the temperature of the Earth's atmosphere. Allcombustion processes involving hydrocarbons including this processproduce carbon dioxide as a byproduct. Virtually all combustionprocesses emit carbon dioxide into the atmosphere as a pollutant.Currently many regulatory agencies including the United StatesEnvironmental Protection Agency (US-EPA) are promulgating regulations toremove or sequester the emitted carbon dioxide from combustion sources.

This invention uses cryogenic (low temperature) collection of the carbondioxide to remove it from the gas stream. A commercially availableprocess compresses and cools the gas stream, causing the carbon dioxideto precipitate as a liquid where it is separated and collected. Thesmall amount of gases remaining after collection of the carbon monoxide,primarily unreacted sulfur dioxide and unreacted carbon monoxide arereturned to the combustor in the diluent gas.

Most of the collected liquefied carbon dioxide is utilized as acommodity but a small percentage is reused as a diluent gas in thecombustion process to moderate the combustion temperatures created bythe use of pure oxygen as an oxidant.

Monitoring of Pollutants

Regulations in all states and in most countries require a facility thatemits air pollutants into the atmosphere to demonstrate that they arecomplying with the applicable air emission standards. To demonstratecompliance, a facility must usually install a system that continuouslymonitors the quality of the gas being released into the atmosphere. Thesystem is called a Continuous Emission Monitoring System (CEMS). This isnot required for this invention since no air pollutants are emitted intothe environment.

Combustor Control Description (FIGS. 9A, 9B and 9C)

The combustion control process is a series of nested control loops thatprovide the necessary regulation of heat production.

The primary loop that controls the heat production is regulated by thequantity of fuel that is admitted to the combustor. The fuel feed musthave a wide range of quantities due to the variety of fuels used in thecombustor.

Within the primary loop are combustion control loops that regulate thecombustion process in Stages 1 a, 1 b and Stage 2. This is controlled bythe speed of the combustion belts and the quantity of oxidant anddiluent added to the fuel as it is combusted. The goal of these controlloops is to have the fuel completely consumed while maintaining therequired pollution control.

All of the components controlled in the combustion system containfeedback to inform the control system if a component malfunctions.Different component types use different types of feedback; for example,the air control dampers include a position sensor so that the damperposition set by the controller is returned to the controller. If theposition of the damper differs from the setpoint, the operator isinformed and if the error is beyond a certain limit, the combustor isshut down.

Where possible, the control system is designed so that minor componentmalfunctions are either self-corrected or the programming compensatesfor the error. If minor errors are noted by the control system, thesystem operator and system maintenance personnel are notified for repairor replacement. This gives the control system a great deal ofintelligence including, where possible, predictive failures.

In summary, the following parameters are used to control the combustionprocess:

Stage 1 a:

-   -   Solid fuel feed rate,    -   Oxygen and diluent flow rate,    -   Firebelt 30 speed,    -   Gasification temperature,    -   Draft pressure,    -   Infrared flame strength,    -   Ultraviolet flame strength.        Stage 1 b:    -   Oxygen and diluent flow rate,    -   Oxygen and combustible gases concentration,    -   Firebelt 31 speed,    -   Oxidation temperature,    -   Draft pressure,    -   Infrared flame strength,    -   Ultraviolet flame strength.        Stage 2:    -   Oxygen and diluent flow rate,    -   Oxygen concentration,    -   Carbon monoxide concentration,    -   Firetube draft pressure,    -   Firetube temperature,    -   Infrared flame strength,    -   Ultraviolet flame strength,    -   Boiler draft pressure,    -   Boiler temperature.        List of Figures and Components

-   FIG. 1—Process Description

-   10—air separator unit

-   11—gas storage for oxygen

-   12—gas storage for nitrogen

-   13—gas storage for argon

-   14—a solid fuel storage unit

-   15—fuel transport conveyor

-   16—inlet feed conveyor

-   17—fuel metering and rotary lock unit

-   18—gasifier

-   19—gasifier firetube with connection to boiler

-   20—boiler

-   21—cyclone

-   22—baghouse

-   23—acid gas scrubber

-   24—carbon dioxide scrubber

-   25—carbon dioxide storage system

-   26—control system

-   FIG. 2—Air Separator Unit 10

-   2-10—turbine compressor

-   2-11—condensing column

-   2-12—fractionating tower

-   10—air separator unit

-   11—gas storage for oxygen

-   12—gas storage for nitrogen

-   13—gas storage for argon

-   FIG. 3—Solid Fuel Storage Unit 14 & Inclined Conveyor 15

-   3-10—walking floor trailer

-   3-11—bin conveyor

-   3-13—walking floor

-   3-14—monitor or sensor

-   3-15—monitor or sensor

-   3-16—monitor or sensor

-   3-17—monitor or sensor

-   15—fuel transport conveyor

-   16—inlet feed conveyor

-   FIG. 4—Surge Bin & Rotary Air Lock

-   4-10—bin feeder

-   4-11—bin feeder chute

-   4-12—monitor or sensor

-   4-13—load cells (4 ea.).

-   17—rotary air lock

-   FIG. 5—ZEF Combustor & Boiler

-   5-10—boiler

-   5-11—firebelt 1 oxygen feed

-   5-12—firebelt 1 diluent feed

-   5-13—firebelt 2 oxygen feed

-   5-14—firebelt 2 diluent feed

-   5-15—firetube oxygen feed

-   5-16—firetube diluent feed

-   30—firebelt 1

-   31—firebelt 2

-   FIG. 6—Cyclone & Baghouse

-   6-10—cyclone

-   6-11—baghouse

-   6-12—cyclone inlet temperature sensor

-   6-13—cyclone inlet pressure sensor

-   6-14—cyclone outlet temperature sensor & baghouse inlet temperature    sensor

-   6-15—cyclone outlet pressure sensor & baghouse inlet temperature    sensor

-   6-16—baghouse outlet temperature sensor

-   6-17—baghouse outlet temperature sensor

-   6-18—cyclone rotary air lock

-   6-19—cyclone eductor

-   6-20—baghouse rotary air lock

-   6-21—baghouse eductor

-   6-22—baghouse bags

-   21—cyclone

-   22—baghouse

-   FIG. 7—Acid Gas Scrubber

-   7-10—acid gas scrubber

-   7-11—caustic spray system

-   7-12—plastic or ceramic packing material

-   7-13—demister

-   7 14—spent slurry collector

-   7-15—pH sensor

-   23—acid gas scrubber

-   FIG. 8—Carbon Dioxide Scrubber

-   8-10—carbon dioxide scrubber

-   8-11—turbine compressor

-   8-12—condensing column

-   8-13—fractionating tower

-   8-14—vent to combustor

-   24—carbon dioxide scrubber

-   25—liquefied carbon dioxide storage vessel.

While the invention has been described in relation to preferredembodiments of the invention, it will be appreciated that various otherembodiments, adaptations and modifications of the invention will bereadily apparent to those skilled in the art.

1. A system for converting solid biomass material to usable heat energyand related byproducts comprising in combination: an air separator forseparating oxygen from air and providing a supply of oxygen, a solidfuel combustion chamber having a first chamber portion with an airlockinlet feed for feeding a metered amount of solid fuel thereto, saidfirst burner stage having a first traveling conveyor firebelt and meansfor feeding oxygen from said supply along the length of said firsttraveling conveyor firebelt, a second burner stage having a secondtraveling conveyor firebelt and means for feeding oxygen from saidsupply along the length of said second traveling conveyor firebelt, athird burner stage constituting a steam boiler having a firetube forcollecting fuel gases from said first burner and said second burnerstages and combusting the collected fuel gases with oxygen from saidseparator and means to generate steam from the heat of combustion insaid third burner stage and a heat utilization device connected to saidsteam boiler.
 2. The system defined in claim 1 including means to add acontrolled amount of a nitrogen-free diluent gas to oxygen in thecombustion chamber and to moderate the burning temperatures to preventoverheating of the refractory and boiler components.
 3. The systemdefined in claim 2 wherein said diluent gas is carbon dioxide from saidseparator.
 4. The system defined in claim 3 wherein the amount of saiddiluent and addition is controlled by the firetube and boilertemperatures, respectively.
 5. A zero emission solid fuel fed combustionsystem comprising an air separator for separating oxygen from air andproviding a supply of oxygen, a solid fuel combustion chamber having afirst chamber portion with an airlock inlet feed for feeding a meteredamount of a solid fuel thereto, a first burner stage having a firsttraveling conveyor firebelt, means for feeding oxygen from said supplyin progressively increasing proportions along the length of the firsttraveling conveyor firebelt, a second burner stage having a secondtraveling conveyor firebelt fed with oxygen from said supply in aprogressively decreasing amount along the length of said secondtraveling conveyor firebelt, a third burner stage constituting a steamboiler having a collector for collecting fuel gases from said first andsecond burner stage and combusting the collected fuel gases with oxygenfrom said separator and means to generate steam from the heat ofcombustion in said third burner stage, and a utilization deviceconnected to said steam boiler.
 6. A solid fuel fed gasification systemcomprising a first chamber having an airlock infeed for feeding ametered amount of solid fuel thereto, a first burner stage having afirst traveling conveyor firebelt with a first oxygen-introducing meansfor introducing a metered amount of oxygen in progressively increasingproportions along the lengths of said first traveling conveyor, at leastone additional burner stage having at least one additional travelingconveyor firebelt with a second oxygen introducing means for introducingoxygen. in progressively decreasing amounts along the length of saidfurther traveling conveyor firebelt, a steam boiler connected to saidgasification chamber for receiving gaseous products resulting from thecombustion of said solid fuel, means for introducing oxygen into saidboiler for converting the thermal energy to steam, a cyclone andbaghouse for particulate collection and a scrubber for removing noxiousgaseous from the exhaust stream.
 7. In a biomass combustion system forconverting biomass to energy and useful products, a combustion methodcomprising feeding said biomass into a combustion chamber through anairlock and supplying oxygen and a nitrogen-free diluent to said biomassto control the combustion process in said combustion chamber.
 8. Themethod defined in claim 7 wherein said nitrogen-free diluent gas isselected from carbon dioxide and argon and mixtures thereof.
 9. Themethod defined in claim 7 including the step of collecting and passingcombustion gas from said combustion chamber through a firetube andadding more oxygen and a diluent gas to control the burning of saidcombustion gas to produce heat, supplying said heat to a boiler toconvert the heat to steam.
 10. The method defined in claim 9 includingpassing residual gases through a cyclone to remove and collect coarsefly-ash and supplying any residual exhaust gas to a baghouse to removeand collect fine fly-ash therefrom and feeding gases from said baghouseto an acid gas scrubber to collect and remove said gas salts, andfeeding gases to a carbon dioxide scrubber to remove carbon dioxidetherefrom and feeding the carbon dioxide from said carbon dioxidescrubber to a liquifier to liquify carbon dioxide and feeding saidcarbon dioxide from said carbon dioxide scrubber to said firstcombustion stage to as a diluent gas.